Orthopaedic implants are becoming increasingly prevalent as millions of patients have been treated for degenerative diseases and other conditions that affect proper hip, knee, shoulder and other joint function. Surgery to replace a joint that articulates in a socket often involves removing the damaged parts of the relevant joint and replacing them with prosthetic components.
For example, consider the hip. The hip joint is often called a ball-and-socket joint because the spherical head of the thighbone (femur) moves inside the cup-shaped socket (acetabulum) of the pelvis. To duplicate this action, a hip replacement implant typically has a stem, which fits into the femur and provides stability; a ball, which replaces the spherical head of the femur, and a cup, which replaces the worn-out hip socket. The cup typically features an acetabular shell and a liner.
Each component of the implant is typically provided in various sizes in order to accommodate different body sizes and types. In some designs, the stem and ball are one piece; other designs are modular, allowing for additional customization in fit. Typically, for cementless applications, a shell is implanted into the socket and a liner is implanted into the shell. A modular prosthetic stem is then implanted into the patient's intramedullary canal and a head (or ball) is positioned on the stem. The head is also adapted to be positioned in the liner, so that the prosthetic is allowed to articulate in the liner, just as a bone would articulate within a natural socket.
In choosing the joint implant components to use, a surgeon takes into consideration many factors, such as the patient's age, weight, and activity level, as well as relevant factors relating to the implant itself, such as the type of liner to be used (e.g., ceramic, cross lined polyethylene, ultra high molecular weight polyethylene, metal, and so forth) in conjunction with the type of shell (and thus the locking connection featured by the shell) to be used. The liner and shell are typically chosen together because connections between liners and shells are material-specific and design-specific.
For example, ceramic liner to metal shell (and metal liner to metal shell) connections typically use a Morse taper connection, which means that the outer surface of the liner is tapered slightly in order to cooperate with a corresponding taper on the inner surface of the shell. This allows the liner to lock in the shell via a secure connection formed by the tapers.
By contrast, polyethylene liner to metal shell connections typically use a non-Morse taper connection because of the low push-out force resistance of polyethylene. In other words, a Morse taper connection may not secure a polyethylene liner to a metal shell because a tapered polyethylene liner wants to “push” itself out of the shell. Polyethylene liner to metal shell locking connections will vary, but two common examples are axial locking features and rotational locking features.
However, there may be instances in which the surgeon would prefer to select the liner and the shell independently from one another, e.g., a particular shell may have a preferred bone in-growth feature or a certain implantation feature, or a particular liner may have properties that will be advantageous to that particular patient. A surgeon may not want to use the type of liner that is adapted to cooperate with the particular shell chosen and vice versa. One work-around method that has been used by some surgeons is to apply cement to the pre-existing shell to secure a polyethylene liner. However, there is not currently a system available that provides such flexibility.
Additionally, although many advancements have been made to prolong the life of implants, joint implant surgeries may need to be repeated due to the wear experienced by the artificial joint over prolonged periods of time due. Wear debris can be generated from the articulating movement of the components against one another, the components may loosen, ceramic inserts may fail due to fracture, recurrent dislocation may occur, and so forth. During revision surgery, which is a surgery that replaces a current implant with a new one, the surgeon often needs to remove the shell, liner, and implant and replace them with new components.
One of the problems experienced with revision surgeries is that the shell, which may be have integrated into the patient's bone over time, has to be removed because the liner being used does not have connecting features that correspond to those of the current shell and/or because the connecting features of the current shell are going to be damaged during removal of the current liner.
For example, if the surgeon is planning to use a non-tapered liner, but the currently-implanted shell is tapered, there is no way to create a secure connection. For instance, if a surgeon is removing a ceramic liner and chooses to replace it with a polyethylene liner, the surgeon will have to remove the entire shell and replace it with a shell having a different connection mechanism because the polyethylene liner likely will not engage properly via a Morse taper. There is nothing to hold the polyethylene liner in place, particularly because as polyethylene warms up (e.g., due to body temperature), it expands and tends to pop out. As discussed above, one alternative method of securing a non-fitting polyethylene liner in place during revision surgery is to cement the liner to the cup, but that is not optimal.
Another example of why the shell often needs to be removed during revision surgery is because when the surgeon removes the initial liner (the one implanted during the primary surgery), even if the replacement liner has a connection that corresponds with the current shell, there is often a threadform on the inner surface of the shell that is deformed or damaged during removal of the initial liner. Although this threadform may have helped to secure the initial liner, it will no longer be operable to secure the replacement liner due to its deformation. This is particularly a concern if the surgeon wishes to use a ceramic liner during the revision surgery because the deformed threadform may create a raised edge when the initial liner is removed, preventing a new ceramic liner from being properly received by the shell. A ceramic liner being replaced is at risk of either (a) being cracked or fractured on the raised edge of the threadform during replacement or (b) causing a higher risk of fracture during prolonged use from cyclic fatigue against the deformation.
Thus, it would be advantageous to provide a universal liner that can cooperate with a currently-implanted shell to cut down on trauma to the patient due to removal of the shell. (Many shells are provided with a bone in-growth material (such as a porous coating or a biological material) that has encouraged the patient's bone to fuse with the shell. Accordingly, removal of the shell can cause unnecessary trauma to the patient and removal of extra bone, neither of which are optimal.)
It would also be advantageous to provide a universal liner that gives the surgeon more choices of liner to shell connection pre-operatively.
Accordingly, there is a need in the art for a more universal liner connection.